The invention describes a method for making a coating and, more particularly, to a method for making a hydrophobic coating.
It is well understood that the wettability of various materials is dependent on both the physical and chemical heterogeneity of the material. The notion of using the contact angle, θ, made by a droplet of liquid on a surface of a solid substrate as a quantitative measure of the wetting ability of the particular solid has also long been well understood. If the liquid spreads completely across the surface and forms a film, the contact angle, θ, is 0 degrees. If there is any degree of beading of the liquid on the surface of the substrate, the surface is considered to be non-wetting. For water, the substrate surface is usually considered to be hydrophobic if the contact angle is greater than 90 degrees. There are materials on which liquid droplets have high contact angles, such as water on paraffin, for which there is a contact angle of about 107 degrees. Many applications require a hydrophobic coating with a high contact angle of at least 150 degrees, and preferably at least 165 degrees. These coatings with such high contact angles are sometimes referred to as by super-hydrophobic.
The rolling of liquid droplets and the removal of foreign particles depend on both the hydrophobicity of the surface and on the surface roughness caused by different microstructures. The property of super-hydrophobicity has been observed on the petals and leaves of the lotus flower, hence the name “Lotus Effect”. At very shallow angles of inclination or with the slightest wind, water droplets roll rather than flow. The rolling droplets entrain particle contaminants/parasites thereby cleaning them from the Lotus leaf surface. It is now recognized that the fascinating fluid behaviors observed for the Lotus plant, like the rolling and bouncing of liquid droplets and self-cleaning of particle contaminants, arise from a combination of the low interfacial energy and rough surface topography of waxy deposits covering their leaves. Because the Lotus-effect is solely based on the chemical and microstructural nature of the surface, it can potentially be mimicked to produce a self-cleaning surface. This self-cleaning property of materials can have various applications in bio-medical and microfluidic devices, protective layers for semiconductors, anti-corrosion coatings, and films on windows.
Directed motion of droplets is of interest in general to create containerless, surface-tension confined fluidic devices that are non-fouling, easy to clean, and allow transport of highly concentrated fluids with no loss to the walls. The potential to deliver highly concentrated fluid samples will overcome a major current obstacle in dielectrophoretic (DE) separations. The ability to coalesce drops also can provide the means to perform highly controlled reactions upstream of the fluidic analysis and has implications also for flow cytometry.